﻿// Copyright (c) .NET Foundation. All rights reserved.
// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.

using System;
using System.Collections;
using System.Collections.Generic;

namespace Microsoft.AspNetCore.Razor.Language.Syntax
{
    internal static class SpecializedCollections
    {
        public static IEnumerator<T> EmptyEnumerator<T>()
        {
            return Empty.Enumerator<T>.Instance;
        }

        public static IEnumerable<T> EmptyEnumerable<T>()
        {
            return Empty.List<T>.Instance;
        }

        public static ICollection<T> EmptyCollection<T>()
        {
            return Empty.List<T>.Instance;
        }

        public static IList<T> EmptyList<T>()
        {
            return Empty.List<T>.Instance;
        }

        public static IReadOnlyList<T> EmptyReadOnlyList<T>()
        {
            return Empty.List<T>.Instance;
        }

        private class Empty
        {
            internal class Enumerator<T> : Enumerator, IEnumerator<T>
            {
                public static new readonly IEnumerator<T> Instance = new Enumerator<T>();

                protected Enumerator()
                {
                }

                public new T Current => throw new InvalidOperationException();

                public void Dispose()
                {
                }
            }

            internal class Enumerator : IEnumerator
            {
                public static readonly IEnumerator Instance = new Enumerator();

                protected Enumerator()
                {
                }

                public object Current => throw new InvalidOperationException();

                public bool MoveNext()
                {
                    return false;
                }

                public void Reset()
                {
                    throw new InvalidOperationException();
                }
            }

            internal class Enumerable<T> : IEnumerable<T>
            {
                // PERF: cache the instance of enumerator. 
                // accessing a generic static field is kinda slow from here,
                // but since empty enumerables are singletons, there is no harm in having 
                // one extra instance field
                private readonly IEnumerator<T> _enumerator = Enumerator<T>.Instance;

                public IEnumerator<T> GetEnumerator()
                {
                    return _enumerator;
                }

                System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
                {
                    return GetEnumerator();
                }
            }

            internal class Collection<T> : Enumerable<T>, ICollection<T>
            {
                public static readonly ICollection<T> Instance = new Collection<T>();

                protected Collection()
                {
                }

                public void Add(T item)
                {
                    throw new NotSupportedException();
                }

                public void Clear()
                {
                    throw new NotSupportedException();
                }

                public bool Contains(T item)
                {
                    return false;
                }

                public void CopyTo(T[] array, int arrayIndex)
                {
                }

                public int Count => 0;

                public bool IsReadOnly => true;

                public bool Remove(T item)
                {
                    throw new NotSupportedException();
                }
            }

            internal class List<T> : Collection<T>, IList<T>, IReadOnlyList<T>
            {
                public static readonly new List<T> Instance = new List<T>();

                protected List()
                {
                }

                public int IndexOf(T item)
                {
                    return -1;
                }

                public void Insert(int index, T item)
                {
                    throw new NotSupportedException();
                }

                public void RemoveAt(int index)
                {
                    throw new NotSupportedException();
                }

                public T this[int index]
                {
                    get
                    {
                        throw new ArgumentOutOfRangeException(nameof(index));
                    }

                    set
                    {
                        throw new NotSupportedException();
                    }
                }
            }
        }
    }
}
